Nowadays, a discharge lamp (xenon lamp) that emits light upon excitation of a xenon gas (Xe gas) is used as the discharge lamp disposed in the above-mentioned devices. It is known that this discharge lamp has a continuous spectrum from an ultraviolet range to a near infrared range, and particularly has a distribution that is very close to natural daylight in a visible range.
In a manufacturing process of such discharge lamp, an evacuation pipe or air-exit pipe (chip pipe) is connected to a discharge vessel to expel an impurity gas from the discharge vessel and introduce a light emitting gas into the discharge vessel. After the light emitting gas is introduced into the discharge vessel, the chip pipe is melted and removed while the sealing is being maintained (chip off step). Some portion of the chip pipe (hereinafter referred to as “chip portion”) remains on the outer peripheral surface of the discharge vessel.
The above-mentioned discharge lamp filled with the xenon gas primarily emits light from an ultraviolet range to a near infrared range. However, as shown in FIG. 6 (energy diagram of a xenon lamp) of the accompanying drawings, the lamp also emits light in a vacuum ultraviolet range. In FIG. 6, it should be noted that light at the wavelength of 147 nm is hardly emitted because of self-absorption by a gas, but light at the wavelength of 172 nm is relatively easily emitted, and this light is absorbed by the discharge vessel with high probability.
Silica glass that is a material of the discharge vessel experiences an accumulation of ultraviolet-derived distortion and/or stress (distortion and/or stress caused by ultraviolet light) over time as the discharge vessel absorbs the ultraviolet light. In particular, the remaining portion of the evacuation pipe, i.e., the chip portion, has a low mechanical strength because of the structure thereof, and is easy to accumulate the distortion and/or stress. Thus, the chip portion is a portion that is easiest to break upon accumulation of the ultraviolet-derived distortion and/or stress over time. The “distortion and/or stress” is referred to “stress” in the following description.
To address such ultraviolet-derived stress, Japanese Patent Application Laid-Open Publication No. Hei. 6-231732 (Patent Literature Document 1), for example, teaches increasing an OH group (OH radical) concentration contained in silica glass, which is the material of the discharge vessel, to suppress or reduce a damage caused by the ultraviolet light. The ultraviolet-derived stress is generated when the molecular binding (Si—O) in the silica glass is disconnected by the ultraviolet light and a structural change takes place. If the OH group concentration in the silica glass is high, it works to fix (repair) the disconnected molecular binding. As a result, the ultraviolet-derived stress is moderated.
In recent years, the Xe lamp for use in a movie theater emits light with a rated electric power and also changes the electric power on the basis of the use. For example, the lamp must emit light at a higher electric power when it is used to project the 3D movie than when it is used to project the 2D movie. However, when the lamp is used to project the 2D movie, an amount of its optical output is too high, and therefore it is necessary to reduce the electric power to be used by the lamp.
In view of the above-described fact that use of the discharge lamp spreads in a variety of applications nowadays, there is a demand for a discharge lamp that has a wide (large) acceptable range of electric power. For example, there is a demand for a discharge lamp that can properly emit light from a rated value to a 50% of the rated value. When such discharge lamp is designed, the magnitude, the size and other factors of the optical output, the electrodes, the sealing body of the discharge lamp are decided on the basis of the upper limit of the electric power to be used.
However, the extensive study of the inventor of this invention revealed that the chip portion connected to the discharge vessel tends to more accumulate the stress when the discharge lamp filled with the xenon gas emits light at low electric power than when the discharge lamp filled with the xenon gas emits light at high electric power. The inventor considers that this is because the temperature of the discharge vessel (sealing body temperature) during the light emission of the discharge lamp is different.
Specifically, the sealing body temperature becomes high when the discharge lamp emits light upon receiving a high electric power, and the atom migration (atom transfer) becomes active inside the material. This works to moderate (reduce) the accumulated stress. On the other hand, when the discharge lamp emits light upon receiving a low electric power, the sealing body temperature becomes relatively low. This lowers the capability of moderating the stress. As a result, the damage caused by the stress tends to remain in the material. In particular, the accumulation of the ultraviolet-derived stress at the chip portion becomes one of major factors of shortening the life (duration) of the discharge lamp and causing the breakage of the discharge vessel. Thus, an appropriate solution to this problem is desired.